On the visual organs in Lamellibranchia.

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Mittheilungen aus der Zoologischen Station zu Neapal; zugleich ein

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On the Visual Organs in Lamellibranchiata.

By
Benjamin Sharp, Dr. med. et pliil.,

Professor of tlie Lower Tnvertebrata at the Academy of Natural Sciences of PMladelphia, U.S. A

With Plate 26.

Introduction.

My attention was first drawn to this subject by observing a number
of Sole7i vagina which were exposed for sale in large basins at Santa
Lucia in Naples. By passing my hand over the basin I noticed that
those Sole7i upon which the shadow of my hand fell , instantly with-
drew their siphons, while those not touched by the shadow remained as
before, immovable. Obtaining some specimens at the Zoological Station
I repeated the experiment and was convinced that the animals possessed

some power of distinguishing light from darkness. This led me to
examine the minute structure of the siphon carefully and my sear
was rewarded by the discovery of a very primitive form of visual organ.

I

so primitive in fact that the term eye can hardly be applied to it.

I wish here to extend my thanks to the Bavarian Government for
allowing me the use of its table during the months of March, April and
May of 1S83 and also to the director, Professor A. DoHiix, and his assist-
ants at the Zoological Station , while in the pleasant company of the
latter I received much advice and assistance.

Part of the material only came from Naples namely Soleii vagina^

Jacohaeus^ Patella coerulea ^ and Haliotis tuherculata. Solen

ensu ^ T^e?2tis mercenariay Ostrea virginica and 2Iact7*a solichsstma

came from the coast of New Jersey, U. S. A., and Mija areiiaria from
Boston, Mass., U. S. A. The specimens of Anomia epliippium and
Mytihis ediilis I collected at Villafranca, France. I am indebted to the
curators of the Acad, of Nat. Sc, for the use of all other material.

^

448 Benjamin Sharp

J

HistoricaL

In working up the literature of this subject, I find, that with the

Will and Siebold , all the authors have confined their
attention to the asiphonated forms and especially to the well known eye

of Pecten.

J. X. PoLi^ in 1795 was the first to mention the fact that any of

the LameUihranclnata possessed eyes. He pointed out that Ostrea [pec-

ten) jacohaea and Spondylus gaecIeroj)us possessed small bright ocelli

distributed on the edge of the mantle in considerable numbers. He

figures and gives a short account of their external anatomy.

In 1834 R. Garnee- describes the eyes of Pecie?i^ Spondylus^ and

Ostrea as being ))small, brilliant, emerald-like ocelli , which from their

structure, having each a minute nerve, a pupil, apigmentum, a striated

body and a lens; and from their situation at the edge of the mantle,

where alone such organs could be useful, and also placed as in Gastro-^

poda with tentacles, must be organs of visions. As far as this goes it

is correct and is the first description of the minute anatomy of this

organ.

In 1840 two authors added to our knowledge on this subject,
A. E. Grube^ and A. Krohx. The former laid down the law that in-
vertebrate as well as vertebrate animals that are free to move about in
the air or water, possess eyes in order to recognize their surroundings
and to direct their movements^.

He then says that snails possess eyesight and that acephales are
blind and this is owing to their stationary condition. The only two ge-
nera, among the acephales, according to Grube, that can see are Lima
and Pecten and these do not form exceptions to the rule as they have
the power to move about from place to place by clapping their shells.
He then enters into the anatomy of the eye and considers it in the three

forms of Pecten hicohaeuSs P. onercularis and P. varius.

*

^ J. X. PoLi, Testacea iitriusque Siciliae eorumque historia et anatome tabu-
lis aeneis illustrata. Parmae 1795. Vol. H. p. 107, Spo7idijlus Tabula XXII fig. 1,
2, 4 et 5. Pecten Tabula XXVII fig. 5, 14 et 15.

- R. Garner, On the nervous system of molluscous animals. Trans, of the
Llnnean Society, Vol. XVII. Ft IV. p. 485 et seq. London 1834. p. 488.

3 A. E. Grube , Uber Augen bei Muscheln. MIjller's Archiv fiir Anat. und
Physiol. Berlin 1840. p. 23 ff.

^ )>Dass Thieren mit ft^eierer Ortsveranderung in Liift oder Wasser Augen
gegeben sind, um ihre Umgebungen zu erkennen und danach ihre Bewegungen
einzurichten.K

On the Visual Organs in Lamellibranchiata. 449

A. Krohn's^ account is much fuller as to anatomy of the eye of
Pecten and goes far ahead of that of any of his predecessors. He was
the first to point out the existence of two nerves passing to the eyeball,
these two being the splitting of the main nerve in the pedicle of the eye.

In 1844 Will 2 investigated a number of LainelUWanchiata and
found that a great many of them possessed eyes of great complexity.
I have , unfortunately , been unable to consult the original article and
know it only through Troschel's Jahresbericht for 1844^, and what
subsequent authors have quoted or made reference to.

C. Th. v. Siebold in his Comparative Anatomy of the Inverte-
hrata^ sustains the observations of Will.

It is here said that the eyes of PJioIas^ Solen^ Vemis and Mactra
are not on pedicles, but situated at the bases of the tentacles. In Car-
ilium there is an extremely large number of contractile pedicles on the
ends of which are the eyes ; Tellina has the edges of the mantle set
with small pedunculated eyes of a reddish yellow color ; Phma has on
the anterior mantle edge forty short pedunculated yellowish brown
eyes , while in Area and Pectimcidus are a large number of brownish
red ones ; A?iomta^ a form not investigated by Will, has twenty brown
sessile eyes situated between the numerous tentacles; in Osti^ea the
visual organs are the most numerous , and the most distinct eyes are
found in Pecten.

M. DuvERNOY^ in 1852 gave a short description of the eye [of
Pecte7i maxirmis , but advanced nothing new on this head and did not
enter into the histology at all.

1 A. Krohn, Uber augenahnliche Organe bei Pecten imd Spondylus. MuL-
ler's Arcliiv fiir Anat. unci Physiol. Berlin 1840. p. 371 flf.

2 Will, Uber die Angen der Bivalven imd Ascidien. Froriep's Neue Noti-
zen a. d. Gebiete der Natiir- und Hcilkunde. Bd. XXIX. Weimar 1844. p. 81
und p. 99.

3 Archiv fiir Naturgeschichte. XI. Jahrg. Bd.II. Berlin 1845. p. 297 : mWill
stellte Untersucbungen iiber die Augen der Muscheln an. Er fand dieselben sebr
hocb organisirt. AuDer bei Fecten, Sjmndj/Ius , Ostrea fand sie derselbe auch bei
Pinna, Area, Peciunculus, 3Iytilas, Canlium, Tellina, IfactrUj Venus, Solen, Pho-

las J zuweilen in ungeheurer Zahl. Auch bei den Ascidien Cynthia, PhaUusia und
Clavellina wurden Augen nachgewiesen , und zwar 14, von denen 8 der Atbem-
und G der Afterrolire angeliuren.«

^ C. Th. v. Siebold, Lehrbuch der vergleicbenden Anatomie der wirbel-
losen Thiere. Berlin 1848. p. 262 u. 263.

5 M. DuvERNOY, Memoire sur le Systeme Nerveux des Mollusques Acepba-
les, Lamellibrancbes ou Bivalves. Mem. de I'Acad. des Sc. de I'lnst. de France.
Tome XXIV. 1854. p. 69 ff.

Mitttieilungen a. d, Zoolog. Station zu Neapel. Bd. Y. SQ

450 Benjamin Sharp

W. Keferstein ^ investigated the eye of Pecten 7naxi77ius and en-
tered into a detailed account of the minute structure and supposes j as
does Leydig^^ that it closely resembles a compound arthropod eye.

V. Hensen^ in 1862 gave by far the most complete account that
we have of the eye of Pectcfi. He investigated the eyes of Pecten jaco-
haeus and Area sp. (?) He entered into a minute account of the anatomy
and histology of the eye a summary of which will be given later in its
proper place (p- 456). He also investigated the forms described as
possessing eyes by Will , but was in the majority of cases utterly im-
able to find any trace of visual organs ^.

In the last edition of C. Gegenbaur's Comparative Anatomy ^ he
says, after considering the embryonic eyes of those mollusca which
are blind in the adult state, that Ai^ca^ Pectimculus^ TeUina^ Pinna etc,
have the organs (eyes) on special optic stalks ; then after referring to
Pecten and Sjjondj/Ius^ he says : »Although these organs have many
peculiarities of structure , they agree in all essential points with the
optic organs of the rest of the 3foUiisca<( .

S. J- HiCKSOX^ in ISSO takes up the eye of Pecten jacohaetis^
P. opercularis and P. inaximus considering carefully the minute ana-
tomy. He differs from Hensen in some points regarding the lens.

E. 0. ScmiiDT*^ in 1SS2 gives a short general account of the eye
oi Pecten sp. (?) and rather doubts the fact of its being purel}" an
organ of vision ^ observing that it is quite possible that it has another

The last paper bearing upon this subject is a short note on the
eye of a species of Spondyhis by S. J. Hickson (1882) ^. He finds that

^ W. Keferstein , Untersuchungen uber niedere Seethiere. § IX. Uber den
Bau der Augen von Pecten. Zeitsclir. f. wiss. ZooL Bd. XIL Leipzig 1862. p. 133 ff.

2 Fr. Levdig, Lehrbnch der Histologie der Menschen und der Thiere.
Frankfurt a. M, 1S5T.

3 V. Hexsex , Uber das Auge einiger Cephalopoden. Zeitschr. f. wiss. ZooL
Bd. XV. Leipzig 1S65. p. 220 ff.

* p. 221 Anm. : »Es ware mir selir lieb geweseu, wenn ich noch einmal frische
Augen hatte priifen kunnen, aber bei erneuter und, wie ich meine, sorgsamer Prii-
fung der Auster und bei friiherer Priifung von Mya und Carclium habe icli trotz
Will's Angaben keine Augen finden k6nnen.«

^ C. Gegenbaur, Elements of Comparative Anatomy. Engl, edition by
F. Jeffrey Bell. London 187S. § 272. p. 353.

^ S. J. HiCKSOx, The eye o{ Pecte/u Quart. Journ. of Micr.Sc. N. S. Vol. XX.
London 1880.

■^ E. 0. Schmidt, Handbuch der vergleiehendeuAnatomie. B.Aufl. Jena 1882.

® S. J. HiCKSOx, The eye of Si)on(lyli(.s, Quart. Journ. of Micr. Sc. N. S.
Vol. XXII. London 1SS2.

Oq the Visual Organs in Lamellibranchiata. 45 1

the eye of this form is essentially the same as that oi Pecten and differs
from it only in a few particulars.

The paper of Delle Chiaje ^ referred to by Hensen ^ and two
short papers of J. Chatin^ referred to by Hickson I have not been
able to find; and have to depend on the references for my knowledge
of them.

The apparent disorder in the following pages may be perhaps ex-
plained here. I first examined in detail the histology of Soleii vagina
and S. ensis. I was then led to take up the other forms, namely:
Ostreaj Mya^ Ve7ius^ Mactra etc. j as I could obtain them, and when
fully satisfied with my results I went through the alcoholic collection
of LamellihrancMata of the Academy of Natural sciences, which I am
sorry to say is not rich, neither are the specimens well preserved, and
I am consequently in doubt as to many forms.

I then arranged the species that I had examined by families in
their order beginning with the lowest; thus reversing the order of
Tryon-^ in his Conchology.

This accounts for the fact that the detailed descriptions are scat-
tered about.

I may here express my thanks to Mr. Tryon for his kind assistance
in determining for me many of the species.

In entering now upon the special part of this paper I intend taking
up in order the different forms of Lamellibranchiata that have been
examined by myself and others. Owing to the bad condition of a num-
ber of the forms that I have examined I can give but fragmentary notes
concerning them.

Taking up first the order Asiphomda I will first consider the

Fam. Ostreidae.

Ostrea vi7^gi7iica Linn. When the anterior border of the mantle of
this species is examined it is found to be fringed with innumerable little
tentacles ranging from 1mm to 2 mm in length and arranged in three

^ Delle Chiaje, Osservazioni anatomiche suH' occhio umano.

2 V. Hensen, Uber das Auge etc. 1865. p. 158 and 222.

3 J. Chatin, BuUetin de la Societe Philomatique. Paris 1877,

* G. W. Tryon, jr., Structural and systematic Conchology: an introduction
to the study of the 3Iolhisca. Philadelphia 1884. Vol. III. p. 116 et seq.

30*

^

452 Benjamin Sharp

irregular rows. Inside of the inner row of tentacles is found a pig-
mented band differing greatly in intensity in different individuals.

I have sought in vain for any appearance of distinct eyes in this
part 5 or in any part , of the mantle ; and further none have been re-
vealed by studying carefully a complete series of sections of the mantle-
border.

It has been asserted by Garner ^ Will and Siebold that Ostrea

possessed eyes of great complexity. They may have examined certain
forms that I have been unable to obtain , but as far as Ostrea virginica
and O. edulis are concerned , I can state that no complex eye exists,
nor any evidence to support their assertions. That these
possess a limited power of vision there is no doubt, as it has been as-
serted by my friend Mr. J. A. Ryder^, a most careful observer, that
when a shadow passes over the extended edge of the mantle it is in-
stantly retracted.

I will now take up the minute anatomy of this part, and endeavour
to point out the seat of vision.

When a vertical section of the anterior border of the mantle is

made we find the free edge thrown into three distinct folds (fig. 1 a, 5, c).
These are evidently longitudinal sections of the tentacles already spoken
of. At the base of the central fold [h] is seen the transverse section
of the large circumpallial nerve (fig. 1 P.N.). This nerve according to
Duvernoy^ comes from the principal or posterior ganglion. In the
same section it can be seen giving off branches into the tentacles. The
circumpallial nerve does not consist entirely of nerve fibres, but ganglion
cells may be seen scattered through it.

The body of the mantle is composed of muscular and connective
tissue fibers , which cross each other in all directions; and imbedded
in them may often be seen distinct nuclei. Between these fibers may be
seen the blood spaces or lacunae (fig. 1 b.s.).

Passing through this mass of connective tissue, perpendicularly to the
circumpallial nerve is seen a bimdle of muscular fibers (fig. 1 r.m.); this
bimdle divides and, when a series of consecutive sections are examined,
they may be seen to become fastened to the epithelial layer that forms
the external covering of the whole mantle. One branch , seen in the
figure (fig. 1 d)j passes to the base of one of these folds. These bands

1 J.A.Ryder, Primitive visual organs. Science. Vol. II. Cambridge, Mass.

1SS3. p. 739.

2 DuvERNOY; Mem. s. 1. syst, nerv. etc. 1854. p. 55.

Oa the Visual Organs in Lamellibranchiata. 453

serve to retract the edge of the mantle within the shell , so that it will
not be injured when the two shells meet.

Besides these retractor muscles are seen the extensor muscles
(fig. 1 e.m.)^ which in the section are cut transversely. This muscle
when it contracts forces the mantle outward. These muscles naturally
are

on the internal surface (fig. 1 B.S.) of the mantle and are

attached at intervals to the shell.

Covering the whole external surface of the mantle is an epithelium
of columnar cells which may be classed under two heads, namely: that
which lies in contact with the shell and cuticle [fig. 1 s.e.)^ and that
lining the mantle which is directed toward the branchial cavity (fig. 1
h.e,). The former [sx.) are much longer and larger, and the surface on
which they are situated is thrown into a great number of folds , these
are probably stretched out when the mantle is fully extended. These
cells are very slender and it is very difficult to distinguish the separate

cells. The nuclei; which have the shape of long sphaeroids, are

situated at the deeper third of the cell and are very small and very
narrow. These cells secrete the nacre which forms the internal layer of

the shell.

The branchial epithelium [h.e.]^ which for us is the most impor-
tant, forms a strong contrast with that lying next to the shell. The
cells forming it are much smaller but more distinct. This epithelium is
not thrown into folds as the other , but forms a continuous layer on the
internal wall of the mantle.

These so-called pallial epithelial cells, however, are not all alike,
for if we examine them closely we find a number of them pigmented.
The cells containing pigment seem to be most numerous and best de-
veloped between the inner two folds (fig. 1 h and c) of the mantle bor-
der. Pigmented cells are also found scattered along the pallial side of
the mantle, and these are the cells which form the black band before
mentioned.

In examining these pigmented cells with a high power we find
them very peculiarly constructed. They are columnar cells possessing
a very large and almost spherical nucleus. At the free end, or that part
directed toward the exterior world, we find that there is a collection of
fine pigment granules of a dark brown color (fig* 2). This pigment
occupies the outer fourth of the cell and does not seem to be of equal
thickness , being thinner in the centre or axis of the cell. The rest of
the cell is filled with a granulated protoplasm in which is embedded
the large nucleus. The nucleus is nearest the tunica (fig. 2 t) on

454

%

Benjamin Sharp

which the cell

is

placed.

As

is seen in the figure

all the cells

are

not of 1

the same shape. There is one set (a) in which we have the pigmented
portion at the broadest part of the cell, the base in fact ; the other part
of the cell tapers to the tunica. These cells possess a nucleus which ap-

pears oval. In the other setj in which the nucleus is spherical; the pro-
portions are reversed , the broad base resting on the tunica j while the
apex is wedged in between the bases of the other set. I am not pre-
pared to say whether the cells of this set contain pigment , as they are
very small and the end that passes outward is so fine , that 5 although I
used a strong immersion lense of Zeiss I was unable to satisfy myself
fully on this point.

On the top or external part of the cell there is seen a transparent
membrane (fig. 2 c) , which seems to be continuous over all the pig-
mented cells. It is of a homogeneous nature j highly refractive ^ and
takes no color in either picro- or borax- carmine. When the epithelium
is separated from the underlying cutis , and the cells that compose it
are a little scattered, this transparent membrane or cuticle, if it may be
so called remains intact , that is , each cell is broken ofi' from it leaving

it unbroken. It may be seen projecting beyond the cells, which are
represented in the figure (fig. 2 c).

On those cells possessing a cuticle no cilia could be detected. A
cuticle is probably formed by each cell and then these separate pieces
become welded together so as to form a continuous whole.

Fraisse ^ found something similar to this in the eye of Fissurella^
that is to say, he found non- pigmented cells between the pigmented
ones and supposed these latter to form the lense and vitreous humor.
But in Ostrea I am inclined to believe that each cell produces the
cuticle which overlies it.

Now it is my opinion that the seat of vision is situated in these
pigmented cells with a highly refractive cuticle. This view is supported
by the fact that similar cells are found in Patella by Fraisse^ where

no lens whatever is present. This point will be considered
later on.

I may call attention here to the so-called epidermis (really
cuticle , fig. 1 cii.) of the shell passing^ in between the external fold a
and middle fold b. This membrane is formed by the cells lining the
internal side of the external fold.

1 P. Fraisse, Uber Molluskenaugen mit embryonalem Typus. Zeitschr. f
wiss. Zool. Bd. XXXV. Leipzig 1881. p. 461.

On the Visual Organs in Latnellibranchiata. 455

As regards the other species that I have examined — Ostrea edidis^
I may say that we have essentially the same state of affairs here as in
O. virginica. I found no evidence of differentiated eyes on the mantle
edge, that have been spoken of by former writers on the subject. This
is probably the form that the European writers have investigated-

Fam. Anoniidae.

A?io72ita ephipjnum has according to Siebold twenty brown sessile
eyes situated between the numerous tentacles. In this as in the pre-
ceding form I was unable to find any evidence of a differentiated eye.

The mantle, which is capable of being contracted to a considerable
distance within the shell, is densely set with tentacles. The upper
border of the mantle is strongly pigmented and thrown into regular
folds; from the bases of these folds spring the non- pigmented ten-
tacles 5 thus causing the pigmented surface to round up , and this is
what Siebold mistook for the sessile eyes. On the lower border^ we

find scarcely any pigment at all, and this side of the mantle forms a
strong contrast with the opposite mantle border. The pigment that is
present in the lower border is collected into little spots lodged at the
bases of the numerous folds into which the mantle is thrown. The ten-
tacles here are also not pigmented and are about as numerous as they
are on the upper border.

These small dots of pigment may also have been mistaken for
eyes by Siebold.

Now by carefully examining a series of sections of the mantle bor-
der I found no trace of ^sessile eyes«. The pigment is black and in very
regular spherical granules deposited only in the epithelial cells. The
characteristic refracting cuticle is present, but not so distinct as in
Ostrea.

*

I have unfortunately no experimental evidence in this species, that
any visual power is present , but from the form of the cells and that
Siebold supposed eyes to be present, I think I can safely assume that
such is the case.

Fam. JPectinidae,

Pecteii. The species of this family have been generally those taken
for the investigation of the eye among the LamellibrancMata , and the

^ I use tlie term upper and lower, instead of right and left for convenience.
The term lower being that which is on the shell, which is attached to the foreign
body.

456 Benjamin Sbarp

anatomy and histology of this complex organ has been well worked
out. In passing it is strange to note that although many authors have
stated that other forms possessed well developed eyes, Pecten liave
always been taken for study (except by Will and Siebold) and no
mention of the anatomy of the eyes of other forms given^ except that

were similar organs.

The following species of Pec^^^i have been studied: P.Jacohaeus^
by PoLi , Grube , Krohn , Hensejst and Hickson ; P. opercidaris by
Grube and Hickson ; P. varius by Grube, and P. maximiis by Duver-
NOY, Keferstein and Hickson. The species studied by Garner is not
given, but is probably P. maximus. Will and Siebold's species are

not given •

As the anatomy and histology of this so-called eye are so well
known, I will only give a brief account of it ; and I may add the spe-

cies P. hastatus and P. 77iagelIaniio the number examined.

The eyeball is placed on a stalk or pedicle, which is covered by a
pigmented layer of epithelium , which extends into the eyeball as far
as the edge of the so-called cornea ; here the epithelial layer looses its
pigment and becomes transparent and forms the anterior part of the
ball ; Hensen did not find this epithelial layer, but thought it might be
present. From the observations of Hickson and Schmidt and my own
preparations I do not doubt the existence of it.

The cells of the pigmented epithelium have a strange form and
are worthy to be taken up and considered in detail. They bear a close
resemblance to the pigmented cells that I have described for Ostrea.
Two thirds of their length is filled with pigment (fig. 15 a and b) and
Hensen says that from some of his preparations he thought that they
might possess cilia (fig. 15 5). He farther adds that the cells give the

impression of being organs for the perception of light. My preparations
show similar cells, although no evidences of cilia are present. It may
be that I have not handled the object with sufficient care.

The eye is made up of, anteriorly the cornea, consisting of the
transparent epithelium resting on a hyaline membrane which is a thin
continuation of the connective tissues of the stalk. The lens w
fills the space immediately behind the cornea is large and made up of
nucleated cells. The lens is considered to be a spherical body by both
Krohn and Keferstein, while Hickson considers it ellipticaL Hensen
considers the cells making up the so-called lens to fill up the space
between the retina and the anterior wall of the eye, making the shape
of an irregular biconvex form. This latter statement I am inclined to

On the Visual Organs in Lamellibranchiata. 457

believe to be the correct one and I am inclined to consider that the de-
finite described shape is due to contraction from the use of reagents ;
and then Hickson says that when removed, in the fresh condition, it is
not elliptical as in hardened sections, which he explains as being due
to relaxation of its ligaments.

From the careful investigations of Hensen and preparations of my
own I am led to believe that the lens fills up the whole space and is
not the elliptical body figured and described by Hickson.

Posterior to the lens is the retina, which extends across the
middle of the eye, perpendicularly to the axis of the eye and stalk. It
is separated from the posterior part of the eye by a thick layer of red-
dish pigment. According to Hickson the retina is made up of five
layers and to Hensen only of three. I do not consider it necessary to
go into details concerning this part , as it differs somewhat in form in
different species.

Posterior to the retina is the tapetum, which gives rise to the
metallic lustre so striking in the living eye. Posterior to the tapetum
and filling up the cavity between it and the posterior wall of the eye-

ball is a mass of red fluid pigment consisting of »a number of bright red
granules«. A layer of cells according to Hensen forms the posterior

part of the eye, the existence of which is denied by Hickson. I have
preparations which demonstrate the existence of this layer and the
distinct cells and nuclei beyond all shadow of doubt; in my case how-
ever the cells are by no means as large nor as tall as those represented
by Hensen, and are not pigmented.

The nervous supply of this organ is very complicated ; two
nerve branches entering at two different points. The nerve comes from
the circumpallial nerve , as do nerves passing to the tentacles between
which the eye is situated. The branch which passes to the organ in
question divides into two branches , one passing into the base of the
ball called by Hickson the complementary nerve and the other he calls

the retinal, which passes up and enters the side of the ball and is
distributed to the anterior surface of the retina.

The question as to the function of this organ is one of considerable
interest. Hickson states that a few experiments have been made on
this subject, concerning the visual power of this animal, and he says :
))It is very doubtful whether they (the so-called eyes) are of much value to
the animal in avoiding its enemies. The most reasonable theory of their
function seems to be that when on the ebbing of the tide , a probability
arises that they will be left high and dry on the shore, they can appre-

458 Benjamin Sharp

elate the fact by the growing intensity of the light, and by that peculiar
flapping motion of the valves the Pectens are so remarkable forj move
away into deeper water« *.

This theory may at once be set aside when we consider that the
Pectens of the Mediterranean^ where we have practically no tide at all,
a state of affairs that has existed for an exceedingly long period of
time, have as well developed eyes as those found on shores where tides
do exist ; and further it seems hardly probable that such a complicated
organ would have developed to determine for the animal, whether it be
out of water or not.

Now I am inclined to believe, as, has been suggested by Hensen^,
that the pigmented epithelial cells that cover this organ and the stalk
on which it is situated , and the only place where we find a marked
deposit of pigment, is the seat of vision for these animals and that the
so-called eye may be an organ of some other sense , as suggested by
Schmidt ^. If this supposition be correct we should have the sense of
vision of the same grade as that of other Lamellibranchs. If the organ
in question is so highly developed , we must necessarily conclude that

vision, if this be the sense, is also highly developed, which does not
appear to be the case. And were this organ an eye of high grade its
advantage to the animal could not be very great, as the food and safety
of the animal does not require such perfected vision. The pigmented
cells (fig. 15) with the refractive ends, seem to answer the limited
vision that they possess.

As regards the complicated organ known as the eyC; I might sug-
gest that , if this be an eye, it is one where we have no true pigmented
layer in any direct relation to either the nerve or the retina. The mass
of red pigment and the metallic-like tapetum would hardly answer the
place of the black choroid coat so essential to the eye.

Fam. Limidae.

The species of Lt?7ia examined by Grube is not given neither does
he consider the anatomy of the eye. He merely states that Pecten and
Li?7ia are the only Lamellibranchs that can see, and then considers
only the anatomy of the eye of Pecten.

The tentacles fringing the border of Lima squamosa j the only

v^

1 S. J. HiCKSON, The eye of Pecten etc, 1880. p. 454.

2 V. Hensen, Uber das Ange einiger Cephalopoden etc. 1865, p. 221.

3 E. 0. Schmidt, Handbuch der vergleich. Anatomic etc. 1882. p. 181

«

On the Visual Orpans in Lamellibrancliiata, 459

species that I liave had opportunity to examine are comparatively very
long, but have no pigment deposited in them whatever ; neither could I
find any trace of pigment at the bases of the tentacles to sustain the
fact that, according to Geube, Lima can see.

the other authors

Fam, Spondylidae.

The investigators who have mentioned that Spondylus possess e} es
are Poli^ Garner, Will, and Hickson. The only one who entered
upon the anatomy and histology was Hickson , who finds the form of
the organ essentially the same as the so-called eye ofPecte7i. The spe-
cies investigated by Poli was
did not give the specific name.

Although it has not been mentioned whether the pigmented cells
of the epidermis of the eye and stalk of Spondylus are capped with a
refractive cuticula , I feel quite sure in stating that such exists , as the
organs are so similar in every respect to those of Pecten. Hickson did
not observe them even in Pecten and consequently did not mention it,
in the short account of Spondylus.

Fam. I^itinidae.

Will

BOLD and further have not had the opportunity of examining any spe-

cimens.

W

SiEBOLD are supported by Gegenbaur, but I am very doubtful, if any
complicated organ is present.

Fam. Aviculidae.

I have had the opportunity of examining a species of Avicida from
Panama [A. sterna?). As far as I know this genus has never been in-
vestigated as regards vision and so I have no experimental proof of
such. The whole edge of the mantle is w^ell furnished with tentacles,
at the bases of which are found well defined pigmented grooves. On
the cone-shaped tentacles about half-way up was found a pigmented
bandj that extended around the cone and in breadth was about equal to
half the height of the cone.

These conish tentacles were comparatively short and measured
from about 1 mm to 2,5 mm in length. I could find no trace of special-

ized eyes. I am inclined to believe that these pigmented grooves and
bands answer the purpose of vision. The animal was not in a fit con-
dition to settle the question by sections, and so I have to conclude from

460 Benjamin Sharp

the general resemblance to other forms where the point is settled
beyond question of doubt.

Fam. Mytilidae.

Mytihs edulis. I have unfortunately not had opportunity to ob-
serve whether this animal retracts the edge of the mantle when a sha-
dow passes over it, but the character of the pigmented cells that line
the edge of the mantle so agree with those forms that have vision, that
I do not hesitate to say that vision exists j especially as this is one of
those forms stated by Will to possess highly complex eyes.

The mantle-edge in this species is not fringed with tentacles j but
is thrown into folds between which the pigmented cells are found. Into
the most external fold as in Ostrea extends the cuticula , which here is
much thicker than in Osirea ; at the base of this fold is attached a
muscle J the retractor. The extensor muscle is also present, but is not
so distinctly marked off into bundles as that described for Ostrea. The
circumpallial nerve is situated a little distance from the base of the 2. fold.

The epithelial covering may here also be divided into two catego-
ries. Those which form the nacre of the shell situated on the external
face of the mantle and the more distinct columnar cells amons: which

are found the pigmented ones, in relatively the same proportion as in

Ostrea.

These pigmented cells are much longer than those of Ostrea being

1

about four times as long but not much broader (comp. fig. 2 and 3)
They have a broad base which is directed outwards, then tapers to nearly
a point where they rest on the tunica (fig. 3 t) . The pigment ,
is of a dark brown color, occupies about one fifth to one sixth of the
. The nuclei are large and distinct and are situated in the granular
protoplasm of the cell near the pigment, that is^ in the upper second
fifth of the cell a little above where the commencement of the tapering
begins.

We find in Mytihis as in Odrea between the pigmented cells an-
other set of cells ; which have their broad ends resting on the tunica
(fig. 3 t) , while the narrow pointed end extends up between the large
bases of the pigmented cells. Here also as in Ostrea I could not satisfy
myself, whether these pointed ends were pigmented or not. This state
of affairs seems only to exist when the seat of vision is on the mantle-

1 The figures 2, 3, 5, 8, 9 and 16 are all drawn with a camera lucida of Zeiss

with obj. Immersion No. 3 and ocular No. 3, in order they would all be on the
same scale to compare the sizes.

On the Visual Organs in Lamellibranchiata. 4C1

edge , while no trace of this second set of cells is present in the forms
with siphons.

These latter cells are filled with a granulated protoplasm

}

seems to take less of the staining fluid than that of the pigmented cells,
so that a contrast between these two sets of cells is formed. The nuclei
are spherical and are placed in the broad ends of the cells, between the
tapering ends of the pigmented cells.

Those pigmented cells, as well as the other cells that line the wall
of the branchial chamber , I feel convinced are ciliated , and I am in-
clined to the same opinion as regards the corresponding one in Ostrea^
but I could not proove it entirely to my satisfaction , and consequently
will not state that such is the case. Those cells situated in the folds
then do not seem to have any trace of cilia.

Dreisse?isia polijmorpha. Here also I am sorry to say that I have
no experimental evidence of vision, but the anatomical structure agrees
so exactly with other forms, on which I have experimented that I do
not hesitate to say, that vision is present.

The mantle in this species is closed and with only a small byssal
orifice; the siphon is only indicated by two specialized openings at the
anterior part of the animal, in which considerable pigment is deposited.

When sections of these simple siphons are made, large and clearly
defined cells are seen , which arc somewhat smaller but much broader
than those found on the mantle-edge oi Mtjtihis. On those cells, in
which pigment is found, a clear and refractive cuticle is seen ; this cu-
ticle is greatly developed and exceeds, in fact, that of any other Lamelli-
branch that I have examined (fig. 16 c]. The nuclei are long and large
and the pigment is deposited between them and the cuticle (fig. 16).

These pigmented cells are deposited in a groove , which extends
around the mouths of the siphons.

Fam. ArcuTae.

Area mco7igrua was found to possess no trace of a deposit of pig-

ment in the mantle. Will, however, states that Area has complicated
eyes on the mantle , but I do not know what species he examined. In
Area amerieana there are crescent-shaped deposits of pigment at regular

vals a little above the mantle-edge on its exterior surface, no other
trace of visual organs are present.

Pectuneuhis sp. (?) The fact of this form possessing highly developed
organs of sight, is known to me only through the works of Will,
SiEBOLD and Gegenbaur ; Siebold stating that numerous brownish-

462 Benjamin Sharp

red eyes were present in both Af^a and Pectunciihis . Gegenbaur also
sustains the fact that the genera possessed eyes.

Fam. Cardiidae.

I have had the opportunity of examining macroseopically the
mantle-edge of Cardmm muricattim , C. edide and C. 7nagnum. Will
and SiEBOLD , both state that Cardmm possesses eyes , the latter goes
so far as to state, that the shining eyes are situated on the ends of con-
tractile tentacles ; which fringe the external openings of the two short
mantle-tubes (siphon) and are extraordinarily numerous. I do not know
the species examined by these authors j but I can state as regards the
species (7. edide^ C. miiricatum and O. magnum , that not the slightest
trace of any such organ was present, and further I could find no trace
of a deposit of pigment. It is true^ that the specimens that I examined^
had been in alcohol for a long time and the deposit of pigment or small
organs may have become destroyed.

Fam. yeneridae.

Venus mer cellar la. When the living siphon is extended, it projects
but a few millimeters beyond the edge of the shell. When the siphon
is cut open, we find that the internal length is about 1,5cm. The ex-
ternal surface of the siphon is brownish and toward the orifices of the
siphons it becomes peppered with black pigment being quite black at
the very end. Internally the siphon presents a yellowish color also
blackened toward the exterior opening.

If the mouths of the two apertures of the siphon be examined,
they will be found to be fringed with fine processes or tentacles measur-
ing about 1 mm in length. These tentacles are most numerous and best
developed at the edge of the branchial siphon and very scarce and small
at the anal. A black line is seen to extend on the partition which divides
the two siphons.

When sections are made of the end of the siphon , so that the
wall is cut through parallel to its greatest length, we find on both
sides of the section , that is the external and internal surface of the
siphon y a layer of well developed and distinct columnar cells. This
layer of epithelium is thrown into numerous folds as is seen in fig. 4.
This layer of cells lies on the cutis , which is made up of connective
tissue and muscular fibers with distinct nuclei. In the middle of the
wall of the siphon are seen numerous parallel fibres, which, in connec-
tion with the retractor muscle, serve to withdraw the siphon.

On the Visual Organs in Lamellibrancliiata,

Among the columnar epithelial cells of the siphon are found pig-
mented ones scattered about in irregular groups, these are especially
numerous near the external orifices of the siphon. The pigmented cells
seem to be of the same size as those, which contain no pigment, and
both are covered with a transparent covering, a cuticle.

The pigmented cells of Venus are by far the longest of any that I
have examined (fig. 5) • In shape they are similar to those of Mytilus^
the cuticle however is much thicker in proportion. In Venus however
we do not have any of the cells that lie between the tapering ends
of the cells. A space is consequently left here, which is due probably
to the contraction of the protoplasm , the pigmented part of the cell
being more dense, does not contract so much.

The nuclei of these cells are very long and narrow and more or
less surrounded with the pigment (fig. 5) .

Venus is another of the forms in which Will found very complex
organs of sight. I however have sought in vain for such organs ; but I
can assert that the animals perceive shadows , which pass over the ex-
tended ends of the siphons and they rapidly contract them.

Ve?iiis verrucosa is another species, that I have had opportunity to
examine, and found the end of the siphon much pigmented and dark
pigmented grooves at the bases of the tentacles.

In Tapes decussata well developed pigmented spots were found
at the extremity of the siphons.

Fam. JPetricolidae.

Petricola lylioladifor^nis. Through the kindness of Mr. Fokd, I have
been able to examine several fresh specimens of this species. The
siphons are large, and the branchial and anal siphons are not joined
at the extremity. The ends of the siphons are more or less pigmented
at their external openings, these being more on the branchial than anal
siphon.

When sections are made the cells forming the pigmented spots
are found to be essentially the same as the pigmented cells already
described. These cells differ, however, in the fact that the pigment is
more or less collected at that part of the cell nearest the base, that is,
where the cell is attached to the tunica. Here the pigment seems to be
deposited around the nucleus leaving the clear granulated protoplasm
between the pigment and the transparent, homogeneous cuticula. The
pigmented epithelial cells as a rule are placed at the base of slight

\

thus being more or less protected.

464 Benjamin Sharp

In Saxidonms Nutallii (Conrad) the siphon is well developed and
darkly pigmented at the extremity.

Fam. Tellinidae.

Will states that Tellina is one of the forms, which have well

developed eyes. I can safely say from the specimens j that I have exa-
mined of TeJlma tenta , that I found not only no trace of eyeSj but also
no pigment deposit present.

Fam. Mactriflfte.

With Mactra soUdissima I have had the opportunity to experiment
and found the same sensitiveness to light and shadow that I have found
in other species, with which I have experimented.

The siphon is well developed, not very long but quite robust. The
external mouths of the siphon are densely fringed with tentacles from
1 mm to 255 mm in length, these tentacles are short and thick and really
have more the appearance of cones than of tentacles.

When sections of this part are made and examined , we find no
essential difference from those already described. The cells contain a
brownish black pigment and are about the same length as those of
Sole7i (fig. 9) , the pigment being deposited next to the well marked
and transparent cuticle. The pigmented cells here seem as in the pre-
ceding form to have a tendency to form into grooves or at the bases
of the tentacles for 2)rotection , this being a step in advance of those
•ms , where the pigmented cells are simply scattered in the general
ium.

In the other species that I have examined, Mactra ponder osa ^ the
grooves were pale , but distinct at the base of the tentacles , and the
siphon tolerably well developed.

Fam. Myacidae.

In Mya arenaria the siphon is from one to one and a half times
the greater length of the shell ; the whole is covered with a compara-
tively thick cuticula, which is thrown into innumerable folds, when the
siphon is contracted. This cuticula is a continuation of the cuticle,
which covers the thick border of the mantle and the newly formed bor-
der of the shell. The cxiticula is thinnest near the external end of the
siphon.

The end of the siphon is quite black and pigment in spots occurs
for quite a distance below the end. The mouths of the siphons are sur-

On the Visual Organs in Lamellibranchiata.

rounded by tentacles , which are more distinct than in Ve7ms mercer
naria\ there are three or four rows of tentacles the smallest outside and
the largest inside , the outer ones measuring about 1 mm in length,
while the inner are about twice as long (2 mm) .

At the base of the inner and longer row of tentacles is seen an in-

tensely black band , which passes around on the internal wall of the
siphon, and is present in both the anal as well as the branchial siphon;

_ ■

at the basis of the other tentacles are seen little lines of black pigment.

When a longitudinal section of the wall of the siphon is made and
somewhat magnifiedj we have a figure as is represented in fig. 6. At a
we have a section of the black band lying at the base of the internal
row of tentacles ; c is one of the external tentacles showing a patch or
groove of pigmented cells at each side of the base,

The substance of the siphon is seen made up of muscular fibers
and connective tissue between the bundles of which are seen the lacu-
nae (fig. 6 /).

When the patches of pigmented cells are more strongly magnified,
we find that they are of greater length than the epithelial cells, which
are not pigmented (fig. 7). We find that, as in the case of Mactra^ the
pigmented cells are concentrated into groups, where they are protected
as at the base of the tentacles , or inside the siphon, but few are ex-
posed at the external wall.

The cuticula which ex

over the cells, is thickest on those

y

cells, that are farthest from

be thickest, or most opaque,

the edges of the groove or patch of cells
(fig. 7) , it gradually diminishes in thickness , so that none is present,
where the unpigmented epithelium is reached. The pigment seems to

in the center of each cell, so as to give
the cells the appearance of containing rods (fig, 7) .

When the pigmented cells are examined with a high power, we
find that they are essentially the same as those described for other
species (fig. 8) , save that the pigment instead of being dark brown is
jet black in color.

Fam. Solenidae.

Soleri vagina. This is the form by which my attention was first
directed to investigate the visual organs of La?nelUbra?ichia(a^ and with
which I made the most extended and complete experiments , leaving
not the shadow of doubt, that visual power existed in these animals.

A preliminary account was published on November 6, 1883^,

1 B. Sharp, On Visual Organs in Sohn. Proc. of Acad, of Nat. Sc. of Phila-

ia. 1883. p. 248.

Mittheilungen a. d. Zoolog. Station zu Neapel. Bd. V.

31

466 Benjamin Sharp

giving the results of my observations on this genus. The siphon in this
form is not long in proportion to the length of the animal , as we found
it to be veith Mya^ but well developed. The mouths of the siphon are
beset with short conical tentacles , which are arranged in many rows.
At the bases of the tentacles and well protected from injury are seen
dark grooves, some of them being from 1 to 1,5 mm in length. These
are very numerous and often as many as fifty or seventy five have been

counted in good sized individuals. Will describes Sole7i as possessing
eyes, unless he had reference to the pigmented grooves I could nowhere
detect anything like an eye , especially one as complicated as he de-
scribes, although I have carefully examined the siphon both with a lens
and by making long series of sections of it.

These grooves are distinctly seen with the naked eye. and as these
are the only places , where pigment is found , I think that we are safe
in concluding that the seat of vision , which is undoubtedly present , is
placed in these grooves.

When sections are made cutting the free end of the siphon verti-
cally, we find that these grooves are composed of pigmented cells simi-
lar to those already described for other forms, and as a rule those cells
situated at the base or bottom of the groove are longer than those
placed higher up. Occasionally when a collection of these cells line
only one side of a groove as sometimes occurs , we find that those far-
thest from the edge are the longest as I hare depicted in fig. 14.

When these pigmented cells are highly magnified (fig. 9] , we find
the nuclei almost if not absolutely spherical ; the cells are typical cy-
lindrical cells. The pigment is of a brown color and most dense at the
free end of the cell and gradually diminishing as we approach the nu-
s, the lower parts of the cells are filled with granular protoplasm.
These cells do not seem to contract at the end in which the nuclei are
placed as seems to be the case with similar cells of Venus (fig. 5) and
Mya (fig. 8). Whether this is due to the fact, that Solen was more care-
fully prepared than the other two , or whether it is characteristic in
Vemis and Mya I am not prepared to say.

No nerves could be detected in Sokfi nor in any other form that
I have observed.

As regards Sole7i e?ms I may say, that we have practically the

same state of afi^airs. I could in no way detect any difference. I also
had opportunity to experiment with this species and with just the same
result as was obtained for SoIe?i vagina ; but as to Solen viridis^ a small
form , the siphons ended abruptly and there was no trace whatever of

Oq the Visual Organs in Lamellibranchiata. 467

tentacles nor pigmented grooves. I do not know the habits of this form
and never saw it alive , but from the form of the siphon and absence of
pigment I doubt if it has any power of vision.

Besides the pigmented cells were found other peculiar cells scat-
tered here and there through the epidermis to which I will merely call
attention. These I have represented in fig. 10; they are clear cells
situated between the columnar cells of the epidermis, the nuclei; situ-
ated near the tunica seem to be crescent-shape , this however may be
due to the action of reagents — these cells are tolerably numerous and
could be found in all the specimens that I cut.

Summary.

In all the forms that I have examined , Spondylus and Pecten pos-
sibly excepted , I find that the seat of vision rests in a number of pig-
mented epithelial cells , which have at the free end a fine homogeneous
cuticle; which is highly refractive. Although I could find no evidence
that nerves were distributed to these cells , it is reasonable to suppose
that they are supplied with nerves as are the cells of the general epi-
thelial surface. Now when we compare these cells with those found in
the eyes of Patella , situated at the base of the tentacles , we find here
a similar state of aflfairs. As is shown by Fhaisse i, the eye of Patella

is a simple invagination of the epidermis, forming a sac which is

open exteriorly, and allows free access to the surrounding medium,

. FiiAissE could find no evidence of nerves being distributed to
this organ. The pigmented cells which lined this invagination were

supplied with nerves that were distributed to the general epithelium as
was the case with the Lamellihranchkda. that I examined. The pig-
mented cells, which make up the eye of Patella (fig. 12), have es-
sentially the same form as those found in the Lamellihranchiata. Now
in the LamellihraiicMata we have even a simpler form of visual organ,
than that oi Patella^ for in Patella we have the organs specialized in

er, two, and situation — in the base of the tentacles at the anterior
or oval end of the beast. In the Astphonata the vision is very general-
ized — patches of pigmented cells scattered at large at that portion of the
animal most exposed to light and danger. A somewhat higher grade is
seen in the SipJioriata^ where the pigment is more localized in the lower

1

P. Fraisse, Uber Molluskenauffen etc. 1881.

31*

468 Benjamin Sharp

forms, while much more so in Solen^ Mactra etc- where the pigmented
cells are in grooves at the base of the tentacles and protected by them.

Now the simplest form of organ capable of vision must have pig-
ment to absorb the light and a refractory body , as is exemplified in
the lowest known form of visual organ, the eye spot oi

Eugh

a flagellate infusorian ; here only a part of a cell acts as an organ of

• •

vision.

In Ostrea the conditions necessary for sight are present, a
of cells that contain pigment and have as their external surface a highly
refractive body , the transparent cuticle , and that vision i s present is
proven by experiment^ ; further the fact that these cells are placed only
where they could be of use to the animal. Morphologically eyes should
be at the oral extremity of the animal , where we do find them in
the larvae oi some oi the Lameliibranc/itata^; but the organs of vision
found in the adult are only physiologically eyes — adaptive or ceno-

genetic organs.

are generalized and scattered more or less on the whole
mantle^ but principally at the posterior end , this we find to be more or
less the case in all the Asiphojiata. When a higher stage is reached^
and a siphon begins as in Dreisseiisia pohjmorpha ; we have the cells
concentrated on that part of the mantle (siphon), which extends farthest
from the shell. In Dreissensia and Verms where , when the animal is
disturbed it retracts the siphon wholly within the shell ^ we have the
pigment cells in spots on the surface of the siphon without any parti-
cular protection. In Ilija^ however, where the siphon can hardly be
retracted within the shell, we have the pigment cells protected by being
placed in grooves at the base of the tentacles, where in the rapid with-
drawal of the siphon through the sand , in which the animal lives no
injury to the delicate organs takes place.

A step higher we have the state of affairs in Soleii and Mactra^
when the pigmented grooves are only at the bases of the tentacles,
and the grooves more or less specialized in number and complexity-
A more highly developed or specialized type is Patella^ where the
grooves are specialized to a spherical pit, but still open and bathed
with the surrounding medium.

These are probably the steps taken in the phylogenetic develop-
ment of the eye , first general , then specialized to that part of the

1 J. A. Ryder, Primary Visual Organs etc. 1S83.

2 C. Gegenbaur, Elem. of Coojp. Anat. etc. 187S. § 272. p. 353.

On the Visual Organs in Lamellibrauchiata.

animal where it be of most use, and in the majority of cases this would
be at the oral end of the animal; but in the LcmiellibrancJiiata . where
we have probably a degenerate group and the head atrophied or absent
and not exposed to the light , the morphological eyes have become lost
and other adaptive organs developed ^ where they would be of the

greatest service,

We have therefore in the Lamellihranclnata ^ the obliteration
•through disuse of true and perhaps once tolerably well developed eyes
and the formation of adaptive visual organs either in an early stage of
development or they have reached a state sufficiently high for the uses

of the animal.

In a higher form even than Patella we have an interesting advance
as shown by Fraisse in HaUotis. Here we have a lens present, but
the eye still open anteriorly; the pigmented cells, however, which form

the ball of the eye have no trace whatever of the refractive cuticle
(fig. 13) seen in Patella and the Lamellihranchiata ^ the want being
fjupplied by a distinct lens. Nerves also are so differentiated as to be
seen in this form.

I may add here that I saw no such appearance in the Lamelli-
hrancliiata^ when I made transverse sections of the whole cell as
Fraisse represents for Patella (fig. 11). I am inclined to think that he

an error in stating , that the clear protoplasm extended to the
surface as he represents. I had found that the cells of the Lamelli-
bra?2cMata also were clear of pigment in the center , but by studying
very thin sections and also oblique ones, I came to the conclusion,
that this only existed at the lower part of the cell , when the pigment
became deposited on the sides of the cells, at the ends, however, the
whole was filled with pigment.

End of March 18S4.

Explanation of Plate 26.

Figures 2, 3, 5, 8, 9, 16 are all drawn to the same scale to give their relative sizes.

Figures 11, 12, 13 are copied from Fraisse and figure 15 enlarged twice

from a figure by Hensen. The colored figures stained in borax carmine.
Fig. 1. Radial section (perpendicular to mantle-edge) of the edge of the mantle

of Ostrea virginica: a external, h middle, and c internal fold of mantle's

edge; h.e. epithelium of the external waU of the branchial chamber;

B,S. Branchial side of the mantle; 5.5. blood spaces in the mantle;

cu. cuticle of the shell ; d a branch of the retractor muscle (r.w.) ; e.m.

extensor muscle; P,N. transverse section of the circumpallial nerve;

s.e. epithelium, which lies next to the shell.
Fig. 2. Five pigmented retina cells from the edge of the mantle of Ostrea virgi-

nica (for letters see text p. 453}.
Fig. 3. Five similar cells from Mytilns ediiUs (for letters see text p. 460).
Fig. 4. Part of section of siphon of Venus mercenaria ^ showing the pigmented

Fig. 5. Five pigmented retina cells from the siphon of Venus mercenaria (letters

as in fig. 2).

Fig. 6. Longitudinal section of the external end of the siphon of 3iya arenaria:

a internal band of pigmented cells; b space between external and in-
ternal row of tentacles.

Fig. 7. Magnified view of base of tentacle marked c fig. 6.

Fig. 8. Four pigmented retina cells from the siphon of 3fga arenaria (letters as

in fig. 2) .

Fig. 9. Three similar cells from Solen vagina (letters as in fig. 2).

Fig, 10. Epithelium from siphon of Solen ensis (for explanation see text p. 467),

Fig. 11. Transverse section of retina cells of Patella coenilea. The pigment col-
lected on the walls of the cells (copied from Fraisse, Taf. XXVI fig. U).

Fig. 12. Three retina cells from the eye of Patella coerulea (copied from Fraisse,

Taf, XXVI fig. 12).

Fig. 13. Six retina cells from the eye of Haliotis tubercidata (copied from FraisSE,

Taf. XXVI fig. 13),

Fig. 14. A group of retina cells of Solen vagina.

Fig. 15, Pigmented cells from epithelium of the eye of Pec^en (copied [enlarged]

from Hensen, Taf. XXI fig. 98/ and/').

Fig. 16. Four retina cells from the siphon of Dreissensia polymorpha (letters as

in fig. 2).

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